In a radio communications system, when two user equipments (UE) need to communicate with each other, a sending UE generally needs to transmit data to a base station first, and then the base station transmits the data, which is received from the sending UE, to a receiving UE through a network. Resources required for the sending UE and the receiving UE to send or receive the data may be allocated by the base station. Specifically, before the sending UE sends information or before the receiving UE receives information, the base station indicates control information to the user equipment over a control channel (for example, in an LTE system, a physical downlink control channel (PDCCH)), where the control information includes resource allocation information, a modulation coding scheme, a new data indication, a power control indication, a frequency hopping indication, a precoding matrix indication, and a HARQ process indication. The UE transmits or receives data in a corresponding format on a corresponding resource according to indication information borne on the control channel.
With rapid development of information technologies, a capacity of a communications system needs to be further enhanced to meet high-speed service requirements of the UE. For example, data can be directly transmitted between UEs that are close to each other, which avoids the trouble that the sending UE transmits the data to the base station first and then the base station transmits the data to the receiving UE through a network. That further reduces resource overhead between the UE and the base station.
When direct communication is performed between the UEs, the sending UE and receiving UE also need to use transmission resources in the network when sending or receiving the data. In the prior art, either the UE itself determines available resources through sensing (the sending UE may determine the available resources through sensing and then notify them to the receiving UE; or the sending UE and the receiving UE negotiate to determine the available resources through joint sensing), or the base station or the network allocates available (or potentially available) transmission resources by sending control information to UEs performing direct communication.
However, if the UE senses the available resources by itself, the base station cannot control resource usage of the UE. In addition, because the UE cannot comprehensively understand conditions of other UEs in the network, serious interference may be caused onto communications links of other UEs. If the base station allocates the transmission resources to the UE, the base station can allocate the most suitable resources to the UE and determine proper transmit power according to distribution of UEs in the network, service requirements of the UEs, channel conditions of the UEs, network load, and other factors, and the interference caused by a communications link between the UEs onto a communications link between the base station and other UEs can be controlled by the base station or the network, which prevents the UEs performing direct communication from causing serious interference onto the communications links between the base station and other UEs. However, because the control information generally includes some dynamic (frequently changing) information, the base station generally needs to send the control information to the UE at a very high frequency, which leads to large overhead of transmitting the control information.